In order to improve bending fatigue strength for extending bending fatigue life of a gear, it is typical at present to employ surface treatment to introduce a residual compressive stress layer into a tooth root of the gear. The residual compressive stress layer applied to a root surface may interact with tensile stress generated during the operation of the gear, and thus stress amplitude of the root surface is reduced, which may cause a position subjecting a maximum equivalent stress to move toward an interior of the gear. Finally, generation and propagation of cracks on the root surface can be suppressed.
At present, several types of gear surface strengthening techniques can add the residual compressive stress layer, and may be classified, by strengthening mechanism, into phase transformation strengthening, chemical strengthening and mechanical strengthening. Both the phase transformation strengthening and the chemical strengthening involve a complicated heat treatment process, and heat treatment may cause a large deformation and a poor surface accuracy. Moreover, the phase transformation strengthening and the chemical strengthening involve a complicated treatment process and a limited effect.
The mechanical strengthening mainly refers to shot peening strengthening. The shot peening strengthening is a most widely used gear surface strengthening technique. In the shot peening strengthening, high-speed shots are used to impact the root surface, and plastic deformation of the root surface is induced by an impact force from the shots, and in turn a residual compressive stress layer is introduced. Based on the principle of the shot peening, surface shot peening strengthening such as ultrasonic shot peening and laser shot peening has been developed. In the ultrasonic shot peening, mechanical energy is provided for the shots by an ultrasonic vibration device, and a small amount of shots are hit and rebounded repeatedly in the ultrasonic vibration device for transmitting energy of ultrasonic vibration to the root surface, and thus plastic deformation and a residual compressive stress layer are formed. In the laser shot peening, a gear surface is coated with a special coating, and part of the coating is vaporized and expanded by laser induction, and thus a large impact force is generated to impact a root surface, finally, a residual compressive stress layer is added.
In a shot peening strengthening process, an entire tooth (including an end surface, a tooth surface and a tooth root) of the gear is impacted to be strengthened. Since the tooth root has a complicated geometrical shape and a narrow space, and it is difficult to perform ultrasonic shot peening strengthening treatment at the tooth root described above, the ultrasonic shot peening is not applicable at the tooth root, and has no pertinence to tooth root strengthening and has a low efficiency. In the laser shot peening, the area of a surface to be processed per unit time is limited and a cost is high, and thus the laser shot peening is difficult to apply to industrial production. Thus, there is an issue of a poor effect and a low efficiency in use of a shot peening strengthening technique to strengthen the tooth root.